Cathode ray tube



Nov. 7, 1939. l, sHoENBERG Er AL CATHODE RAY TUBE Original Filed Sept. 28, 1934' 2 Sheets-Sheet 1 INVENTORS I. SHOENBERG, G. E. C0

NDL/FFE A W F. TED/'IAM 7 KLWW ATTORNEY.

Nov, 7, 1939. E-r AL I cATHoDE RAY' TUBE y.1. sHoENBERG 2 Sheets-Sheet 2 original Filed sept. 2s,4 1934 INVENToRs L SHOEA/EERG, 6. E. CoA/DL/FFE F. TED/'IAM ATTORNEY.

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Patented Nov. 7, 1939 UNITED STATES cArnonE RAY TUBE Isaac Shoenberg, London, George Edward Condliffe, Buckinghamshire, and William Francis Tedham, Abbey Wood, London, England, assignors to Electric & Musical Industries Limited, Hayes, Middlese Great Britain x, England, a company oi Original application September 28, 1934, Serial No. 745,838. September 24, 1937,

Great Britain October 3, 1933 4 Claims.

This application is a divisional of our copending application Serial No. 745,838, led September 28, 1934, andv entitled Cathode ray tubes, and now domestic patent numbered 2,119,119.

. The present invention relates to improvements in cathode ray tubes.

A cathode ray tube is known which comprises a cathode, a modulating electrode or modulator, one or more focusing electrodes, an anode and a fluorescent screen. Tubes oi this kind are used for purposes such as television reception, received picture signals being applied vbetween the cathode and modulator so as to vary the intensity of the ray reaching the screen and thus i'. produce a corresponding variation in intensity means. 'I'he present invention relates exclusively to the latter class of cathode ray tube,

It has been found, however, that the modulationcharacteristic curves of hard tubes, that is to say, the curves obtained by plotting modulator Volts (relative to the cathode) vas. abscissae against fluorescent screen current as ordinates, are excessively curved, changes in fluorescent screen current for large changes in modulator volts being disproportionately large compared with the changes in fluorescent screen current produced by smaller changes in modulator volts.

It has been found that the size of the spot varies with the modulator volts, generally increasing as the potential of the modulator approaches that of the cathode, thus giving rise to loss of detail in the reconstituted picture.

It is an object of the present invention to provide means for at least partially eliminating the above mentioned disadvantages'.

It is another object of the present invention to provide an electric circuit comprising a cathode ray tube of the hard type having arranged with its envelope, in the order. mentioned, a cathode, a first accelerating electrode, a deoelerating electrode having an apertured diaphragm Yand a second accelerating electrode and a screen, the accelerating and decelerating electrodes being in the form oi electrodes, so disposed that the ray constituted by electrons from said cathode can pass through them to a screen associated with Divided and this application Serial No. 165,496. In

(Cl. Z50-155) the tube, the tube being provided with electrostatic or electromagnetic means for focusing the ray in a small spot on the screen an'd with means for deiiecting the ray over the screen, and means for applying to said accelerating electrodes potentials positive with respect to the cathode potential and to the decelerating electrode a potential approximately equal to the cathode potential, the.` shape and disposition of the electrodes and the potentials applied thereto being such that, in operation, increases of potential of the decelerator, in the negative sense'with. respect to the cathode potential, produce increases of current to the iirst accelerator. 'Ihe aperture in the diaphragm of the decelerator is of the ordinary type, that is to say, it is ynot obstruc-ted by grid wires' or the like extending across it.

With this arrangement, electrons emitted from the lcathode are slowedr up to a velocity not differing greatly from zero in a region lying in the neighborhood of the decelerating electrode and are accelerated from this region towards said screen. Such a region will for convenience be referred to as a deceleration region.

v In this way there has been produced a cathode ray tube having a nearly straight characteristic, which is desirable for some purposes, and one in which there is reduced change of size of the fluorescent spot` with change in modulator voltage It has, however, been found that with tubes of this kind there may be a halo of light around the iiuorescent spot. It is thus a further object of the present invention to provide a. cathode ray tube in which no such halo is formed around the fluorescent spot. With this end in view an electric circuit comprises a cathode ray tube of the hard type having arranged within its envelope in the order mentioned, a cathode, a rst accelerator, a rst decelerator, a second accelerator, a second decelerator, a third accelerator having an apertured diaphragm and a screen, the accelerators and decelerators being in the form of electrodes so disposed that the ray constituted by electrons from said cathode can pass through them to said screen, the tube being provided with electrostatic or electromagnetic means for focusing the ray in a small spot on the screen and with means for deflecting the ray over the screen, and means for applying to the accelerators potentials positive with respect to the potential of said cathode and to the decelerator potentials approximately equal to the potential vof said cathode, the shape and disposition of the electrodes and the potentials applied thereto being such k,Eli

that, in operation, increases of potential of said irst decelerator in the negative sense with respect to the potential .of said cathode, produce increases of current flowing to the first accelerator.

Tubes constructed in this way may have a characteristic which is curved but to a less eX- tent than the known tubes above referred to and the change of spot size with modulator potential is satisfactorily small whilst the halo is substantially absent. The relatively small but still definite curvature of the characteristic which is obtained is advantageous for some purposes where for example it is desired to increase the detail in the lighter parts of the reproduced picture in comparison with the detail in the darker parts.

Henceforth accelerating and decelerating electrodes will be called, for convenience, accelerators and decelerators respectively.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in. which Fig. 1 illustrates a tube having two accelerators and one decelerator arranged so that there is formed, during the operation of the tube, one deceleration region.

Fig. 2 shows a circuit arrangement for supplying appropriate potentials to the various electrodes of a tube of the kind illustrated in Fig. 1.

Fig. 3 illustrates a tube having three accelerators and two decelerators arranged so that two deceleration regions are formed during the operation of the tube, and

Fig. l shows a circuit arrangement for supplying appropriate potentials to the various electrodes of a tube of the kind illustrated in Fig. 3.

In the embodiment illustrated in Fig. 1, a cathode ray tube suitable for purposes of television reception, comprises an evacuated, sealed glass envelope consisting of a cylindrical portion I flaring out into a frusto-conical portion 2. The diameter and length of the cylindrical portion I are about 1% inches and 8 inches respectively the diameter of the base 3 of the frusto-conical portion 2 is about 9 inches, whilst the overall length of the envelope is about 19 inches.

`Sealed internally to the closed end of the cylindrical portion I in a glass tubular portion 4 which projects internally from and lies wholly within the cylindrical portion I. The tubular portion 4 is about 1/2 inch in internal diameter 'and 61/2 inches in length.`

Disposed within the tubular portion 4 are a cathode heater 4A, a cathode 5, a cathode shield 6, a first accelerator l, a decelerator 8 and a second accelerator 9, these electrodes being arranged within the glass tubular portion ll in the order mentioned. The decelerator 8 is adapted to function as a modulator and the second accelerator 9 corresponds to what is sometimes known as the rst anode of the tube. Leads to the electrodes disposed within the glass tube 4 may be passed through a pinch I sealed into the closed end of the tubular portion I, the leads to the cathode and its heater also serving as supports for these electrodes whilst the cathode shield, the two accelerators and the decelerator are mounted as sliding lits within the glass tubular portion Il. Leads to the three last mentioned electrodes pass from the pinch II] through holes such as II, I2 in the side walls of the glass tube 4 and back through holes such as I3, I4 on to the electrodes. The second accelerator 9, about 3 inches in length, is mounted within the tubular portion 4 in such manner that 1 inch or more Aof the electrode protrudes beyond the tubular Vfrom the pinch I0. All of the electrodes mentioned, except the heater, are of cylindrical shape and are mounted co-axially within the envelope.

A second anode I5, in the form of a cylinder of 31/2 inches length, is also mounted as a sliding fit within the cylindrical portion I of the envelope in such manner that the second anode overlaps the second accelerator or first anode by about 1A; of an inch.

On the inner side of the base 3 of the frustoconical portion 2 of. the envelope is deposited a iiuorescent screen IS of any known or suitable material. Either electrostatic or electromagnetic means such as two pairs of coils I'I, I8 may be utilized to deflect the ray so as to scan the uorescent screen I6 and these means may be situated, either inside or outsidethe tube between the second anode I5 and the screen I6 and close to the second anode.

The cathode 5 may be of any suitable kind but preferably is in the form of a shallow open pillbox of V4 inch diameter, electrons being emitted from the flat circular base which is indirectly heated by means of the heater which lies within the box. The cathode may be coated with an electron emitting substance such as a mixture of strontium and barium oxides.

The shield 6 is also in the form of a pill-box, of about 1/2 inch length and diameter, having a central aperture I9 in its base and being disposed around the cathode 5 so that the cathode lies close to (but does not touch) the bottom of the shield` 5 and. emits electrons through the aperture I9 in the base of the shield.

The rst accelerator l is in the form of a very shallow open pill-box; about 2 millimetres in length and 1/2 inch diameter. It has a central aperture 20 in its base which is disposed about 3 millimetres away from the base of. the shield 6 and the skirt of the rst accelerator is turned toward the cathode 5.

The decelerator 8 is in the form of a short cylinder, about 1/2 inch length and 1/2 inch diameter, with a disc (having an aperture 2l disposed therein) disposed along its length somewhat nearer the first accelerator 'I than the second accelerator 9. One end of the decelerator 8 is about of an inch away from the rst accelerator 1.

The second accelerator or rst anode 9 bears two discs, the one disposed at the end of the electrode lying closer to the decelerator 8 having an aperture 22, and the other being disposed about l inch from the opposite end and having an aperture 23.

The second anode I5 carries no apertured discs, and a lead to lthis electrode may be taken through the wall of the frusto-conical portion 2 of the envelope.

The apertures I9, 2D, ZI and 22 are all of 0.075 inch in diameter, whilst the second aperture 23 in the accelerator 9 is of 0.15 inch diameter.

The various electrodes are maintained at their appropriate potentials by means of the circuit arrangement shown in Fig. 2.. A

An alternating potential difference of any convenient voltage, is stepped up by means of a transformer 34 to 5,000 volts and rectified in the device indicated generally at 35. This steady potential is maintained across a potentiometer .resistance `36, 31.

,i television receiver, the received signals are detected and amplied in a device shown generally at 152. The output from the device 42 is applied across a resistance 43 one end of which is earthed and the other end of which is connected to the decelerator or modulator 8.

It will be seen that the modulator 8 derives a .negative bias from the high tension supply to A the tube and a positive bias from the flow of picture signal current through resistance 13. The resultant bias is such thatthe picture signals operate to vary the potential of the modulator 8, relative to the cathode, from zero down to about -30 Volts, at which voltage the ray is extin-l guished. The potentials above given have been so selected in relation to the shape and disposition `of the electrodes that increases in potential of the decelerator 8 in the negative sense produce increases in current flowing to the first accelerator 1.

It is believed that the operation of the tube is somewhat as follows:

Referring more particularly to Figi, electrons emanate from the cathode e with random velocities and directions. The electrostatic ac celerating field existing between the positive rst accelerator 'I on the one hand and the earthed cathode and shield 5 `on the other hand has the effect of extracting a large number oi electrons from the cathode and concentrating these electrons so as to pass through the aperture 2).!l

i'n the decelerator S.

The decelerator is at a negative potential with respect to the first accelerator 'i so that between -these two electrodes the electrons are decelerated and a deceleration region is formed.

The second accelerator 9 is at a high positive potential with respect to the decelerator 8' and the accelerating held which exists between these two electrodes extends into the deceleration revgion of the tube. The velocities (in the direction of the axis of the tube) with which the electrons emerge from the cathode 5 are small compared to the axial velocities which they'subsevquently acquire owing to the electrostatic accelerating eld existing between the various electrodes, and electrons `which emerge from the cathode with comparatively large transverse velocities V(say up to 0.2 volt) are to a very large extent iiltered out by the decelerator 8 so that the electrons drawn from the deceleration .region toward the second accelerator S have almost the same axial and radial velocities and hence can be brought to a very sharp focus on the fluorescent screen |45. The focusing is effected in the following manner. l

Between the deceleration region and the distant end of the second accelerator or rst anode 9 'the electrons tend to 'diverge owing to the shape of the electrostatic eld's produced between the electrodes. On reaching the comparatively strong Velectrostatic field maintained between the adjacent endsof the first and second anodes, however, the diverging electrons are concentrated or focused back on to the axis of the tube and at the same time accelerated towards and through the second anode l5. The held between these two electrodes acts upon the diverging cone of electrons in muchl the same way as a lens acts upon a diverging beam of light, and

for this reason the arrangement of iirst and;

second anodes may be called an electron lens. In the-present oase the cone of electrons which is diverging on entering the electron lens emerges as a converging cone and is brought to a focus upon the fluorescent screen i6.

a spot of very small size can be obtained upon the screen and that changes in size of the spot,

with changes in modulator volts, are less than served in tubes of known kind.

In the 'second embodiment of the invention the shield E, shown in broken lines in Fig. l, may be omitted and the cathode '5, together with its heater, moved uptowards the hrst Iaccelerator '1 so that there is about 1/8 inch between the base of the first accelerator and the base oi the cathode 5.`

The rst accelerator 'l is in this case maintained at about l0 volts positive with respect to the cathode whilst all other electrodes are maintained atjthe potentials specied for the last described embodiment.

An accelerating field exists between the cathode and rst accelerato-r 'l so that electrons are accelerated towards the' rst accelerator, some electrons passing through the aperture in this electrode.

Between the iirst accelerator and decelerator the electrons are decelerated asthey approach the ydeceleration region which, as beforalies in the pass thro-ugh the aperture le in the shield d,

are` inthe second embodiment attracted towards and collected upon the rstaccelerator 'l so that a iiuorescent spot of comparatively weak intensity is formed uponv the screen i5.

Because of this theiirst embodiment is to be preferred.

It has been found that although with the two embodiments describedabove the change of spot size with modulator (or decelerator) volts is less than that obtained with known tubesthe spot possesses a halo of light. In the embodiment illustrated in Figs. 3 and 4, however, this disadvantage is almost entirely eliminated'.

The tube illustrated in Fig. 3 is of the same general form as that illustrated in Fig. l but includes an extra decelerator and an extra acn oelerator. Thus within the glass tube 24, which corresponds to the tube li of Fig. 1, there are dsposed,. in the order mentioned,v a cathode heater 24A, a cathode 25, ashield 2t, `a rst accelerator 21, a first decelerator, a second accelerator .28, a. second decelerator Sii and a third accelerator or iirst'anode 3 i. A second anode 32 is mounted as va sliding t in the cylinvdrical .portionof .the envelope.

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` 5.1.5 It has been found that with this arrangement The cathode, shield and rst and second anodes are of the same form as those illustrated in Fig. l. The nrst accelerator 2l is in the form of an open pill box of 0.04 or 0.08 inch height, its base, which has a central aperture, being turned away from, and being disposed about 0.08 or 0.12 inch from, the base of the shield 20. The first decelerator 28, which may be utilized as a modulator, is also in the form of an apertured open pill box of about 0.08 or 0.12 inch height, its base being turned away from and being disposed about 0.12 or 0.16 inch from the base of the first accelerator 2l. The second accelerator 29 is of exactly the same size and shape as the first accelerator 2l; its base is turned away from and is disposed about 0.08 or 0.12 inch from, the base of the rst decelerator 28. The second decelerator 30 is in the form of a short cylindrical tube having a centrally apertured disc disposed within it at a point somewhat nearer the end which is adjacent (about 0.04 inch from) the second accelerator 20 than the other end which is disposed about 0.04 inch from the third accelerator or first anode 3i.

The apertures in the shield, accelerators and decelerators are all equal to about 0.08 inch diameter and the diameters of these electrodes themselves are all equal to the diameter of the first anode.

As before, appropriate potentials are applied to the electrodes from a common potentiometer resistance M, 45 across which is maintained a steady potential diierence of 5,000 volts. Relative to earth potential the cathode 25, cathode shield 20 and second decelerator 30 are all maintained at about 40 volts positive, the first and second accelerators 21 and 29 respectively at about 250 volts positive, the third accelerator 3l at about 1,000 volts positive and the second anode at 3,900 volts positive. Received picture signals are used to modulate the intensity of the ray in a manner similar to that described in connection with Figs. 1 and 2, bias for the tube being derived partly from the high tension supply and partly from the flow of picture signals through a resistance 4S inserted in the output circuit of a radio receiver 47. The arrangement is such that the picture signals are effective in varying the potential of the first decelerator or modulator 28, relative to the cathode, from Zero down to about 30 volts at which voltage the ray is extinguished. Over this range of modulator volts no halo and only a small change of spot size is observable. As before, the potentials applied to the electrodes are such that an increase in the potential of the decelerator 28 in the negative sense produces an increase in the current flowing to the first accelerator 21.

During the operation of the tube shown in Figs. 3 and 4, two deceleration regions are formed. Electrons are extracted from the cathode 25 by the relatively positively charged first accelerator 2l and are focused upon the aperture in the modulator or rst decelerator 28. The potential of the modulator varies from Zero to 30 volts relative to the cathode, so that the electrons are slowed up as they approach a deceleration region near the aperture in the first decelerator 28. From this point the electrons are accelerated towards the second accelerator 29 and are focused upon the aperture in the second decelerator 30. This decelerator has the effect of again slowing up the electrons as they approach a second deceleration period which lies in the neighborhood of the second decelerator 30.

The first and secdescribed is dependent upon the relative potentials ofthe various electrodes, their separation, the sizes of the apertures `in them and the density of the current flowing through the tube and the improvement in the constancy and definition of the fluorescent spot is believed to be due to the fact that the second decelerator 30 is not at a negative potential with respect to the cathode 25. l In the tube illustrated in Figs. 3 and 4 modulation of the ray may be effected by varying the potential relative to the cathode, of theshield 20 or of either of the decelerators 28 andv 30, the potentials of those two of these electrodes not used forl modulation being held at approximately cathode potential.

It not essential that the rst and second accelerators 21 and 29 should bemaintained at the same potentials relative to the cathode. It may, for instance, be convenient to keep either or both o them at the same potential as the first anode nl The number of deceleration regions formed in the tube may be greater than two, the formation of each such region being achieved by the introduction of an extra pair of electrodes, one serving to accelerate the electrons and the other serving to decelerate them. l

A magnetic electron lens may be usedto assist in forming a focused spot upon the fluorescent screen in place of the electrostatic electron lens described above.` Moreover a magnetic electron lens may be `used to assist in concentrating the electrons in the neighborhood of the deceleration region. the final focusing on the uorescent screen being thus achieved by means of another electron ens.

If sharp focusing of the ray is not essential the second anode of either of the tubes illustrated (shown in broken lines in the figures) may be omitted.

The characteristic curve of a cathode ray tube can generally be represented by the equation IA=KEgI, Where K represents a constant, IA represents the fluorescent screen current and Eg represents modulator voltage. For the tube just described a: is equal to about 1.8, whilst for most known tubes :c has a value of 2 or more and for the tube illustrated in Fig. l .r is equal approximately to unity.

Over the greater part of the useful current range of the tubes described above the intensity of fluorescence is substantially proportional to the intensity of the current flowing to the fluorescent screen, so that in the equations given above IA may be taken as representing the intensity of iiuorescence as well as the lluorescent screen current.

The index, or gure in the above equations therefore determines what may be called the intensity-contrast'of the tube. An appreciation of what is meant by the expression intensitycontrast may be gathered from the following considerations.

It has long been recognized in the cinema art, that if the brightnesses of all points of an image viewed on a flat projection screen are proportional to the brightnesses on the object from which the images are derived, the screen image appears at 4and uninteresting to the eye. This effect is believed to be due to the fact' that the images are projected in black and white Aand the additional effect of detail which Would be given` to the lighter parts of the picture by the natural colors is absent. It is, therefore, common practice in the cinema industry to develop picture nlms in such manner that detail in the lighter or less opaque portions of thepositive film pictures is brought out, or amplified as it were, relatively to the detail in the darker or more opaque portions of the pictures. A lm developed in this way, and the images projected from it, are said to possess intensity contrasts of value greater than unity and the images derived from the lm are more pleasing and appear more full of detail than images projected from a nlm of intensity contrast equal to unity, that is to say, a film in which the light transmitted through all points is exactly proportional to the brightnesses of the corresponding object points.

lt is found that in fact a lm developed to an intensity contrast of from about 1.8 t0 2.0 is most satisfactory in this respect.

Similarly a tube having an intensity contrast greater than unity is one which amplies signals of large amplitude to a greater extent than it does signals of small amplitude and a tube having an intensity contrast of unity is one which produces no amplitude distortion.

Ii noW the signals received at a cathode ray tube receiver are truly representative of the tone value cf the object, that is to say, are transmitted with an effective intensity-contrast of unity, then, since the reproduced images are to be viewed upon a screen, the receiver tube should have an intensity-contrast higher than unity and usually in the neighborhood of 1.8, because this is usually found to be the intensity-contrast Which app-ears most pleasingto the eye. On the other hand if the signals are transmitted with an eiective intensity contrast greater than unity (say, for example, are derived from a motion picture nlm developed in the usual way so as to have an intensity-contrast of about 1.8), then the tube at the receiver should have an intensity contrast of about unity.

A tube having an intensity-contrast of 2.0| or higher is rarely used, however.

In the embodiments of the present invention Which have been described above it is found that.

if the potential on the modulator be increased negatively with respect to the cathode, the current flowing to the first accelerator increases in such a Way that on plotting first accelerator current as ordinates and modulator volts as abscissae a roughly straight line of negative slope is obtained over the Working range (0 to- 30) of modulator volts.

The values given for the various members of the circuits illustrated in Figs. 2 and 4 are by Way of example and should not be regarded as strictly limiting values.

Having described our invention, what We claim is:

1. A cathode ray tube comprising an envelope having an end Wall, a fluorescent' screen supported on the end wall Within the envelope, and

in the order named, an electron emitting cathode positioned in register with the luminescent screen, a shield electrode coaxial with and surrounding the cathode, a first accelerating cylindrical electrode positioned in register with the shield electrode, a decelerating cylindrical elec` trode positioned in register With the rst accelerating electrode, a second accelerating `cylindrical electrode positioned'in register With the decelerating electrode, said first and second accel- ,erating electrodesv anddecelerating electrodes -drical electrode positioned' in register With the rst decelerating electrode, a second decelerating electrode, a third accelerating electrode, said first, second and third accelerating electrodes and rst and second decelerating electrodes having substantially the same diameter, and an anode .positioned intermediate the fluorescent screen and the third accelerating electrode.

3. A cathode ray tube comprising an envelope having an end Wall, a fluorescent screen supported on the end Wall Within the envelope, and in the order named an electron emitting cathode positioned in register with` the luminescent screen, a shield electrode coaxial with and surrounding the cathode, a iirst accelerating apertured cylindrical electrode positioned in register with the shield electrode, a decelerating apertured cylindrical electrode positioned in register with the rst accelerating electrode, a second accelerating apertured cylindrical electrode positioned in register with the decelerating el-ectrode, said. rst and second accelerating electrodes and decelerating electrodes having substantially the same diameter and the same aperture area, and v an anode position-ed intermediate the fluorescent screen and th-e second accelerating electrode.

4. A cathode ray tube comprising an envelope having an endwall, a fluorescent screen supported on the end Wall Within the envelope, and in the order named an electron emitting cathode positioned in register with the luminescent screen, a shield electrode coaxial with and surrounding the cathode, a first accelerating apertured cylindrical electrode positioned in register with the shield electrode, a rst decelerating apertured cylindrical electrode positioned in register with the first accelerating electrode, a second accelerating apertured cylindrical electrode positioned in register with the first decelerating electrode, a second decelerating electrode, va third accelerating electrode, said nrst,` second and third accelerating electrodes and rst and second decelerating electrodes having-substantially the same diameterv and the same aperture area, and an anode positioned intermediate the iluorescent screen and the third accelerating electrode. 

